A patient's caloric expenditure can provide an estimate of the patient's energy balance and fitness. This measurement can be important for the optimization of therapy provided to patients that have heart problems and require a cardiac rhythm management device such as a pacemaker or a cardiac resynchronization device. For example, caloric expenditure measurements can be used to assess the lifestyle, exertion level, exercise capacity, cardiovascular functional capacity, quality of life, and wellness of a patient for optimization of overall therapy management.
Caloric expenditure is typically measured using the metabolic equivalents (METS) scale. Each MET on the scale corresponds to a multiple of resting caloric expenditure of approximately 3.5 ml/kg/min of oxygen.
There are several known methods for estimating caloric expenditure. One method of estimating caloric expenditure involves indirect calorimetry using cardiopulmonary gas exchange (CPX) variables (VO2, VCO2) of a patient. However, this measurement is difficult to do, particularly outside of a laboratory setting, in that it is necessary to have the patient equipped with respiratory monitoring devices, such as a breathing mask and sample tube, a gas analyzer, ekg leads, and an electronics module for processing the parameters being monitored.
Another method of estimating caloric expenditure is based on the patient's intrinsic heart rate.
For patients that are chronotropically incompetent, other methods for estimating caloric expenditure include use of measurements taken from adaptive rate sensors such as, for example, accelerometers and minute ventilation sensors. For example, U.S. Pat. No. 6,273,856 to Sun et al. describes a method of estimating caloric expenditure using measurements from an accelerometer and minute ventilation sensor used in adaptive rate pacing.
Improvements are needed to methods that allow for estimation of a patient's caloric expenditures during the patient's normal living activity, without requiring difficult, exercise-based CPX measurements.